A Machine-to-Machine (M2M) system is recently attracting attention as a next-generation communication technology. A new IEEE 802.11 ah standard is establishing to support the M2M system even in a IEEE 802.11 WLAN (wireless local area network). A machine rather than a man becomes the subject of the communication and transmits/receives information in a network used in the M2M system. One element of the M2M system may become from the sensors such as a temperature sensor or a humidity sensor to home appliances such as a camera, a TV etc., processing machines in a factory, and large machines such as an automobile.
In recent, as various communication services such as smart grid, E-Health and ubiquitous are emerging, the M2M technology for supporting them has been highlighted. The characteristics of the M2M system are as follows.
1) Many stations: it is assumed that, unlike the existing network, many stations are present in the network used in the M2M system. This is because the sensor etc. disposed at home, company etc. as well as the machine such as a terminal owned by individuals should be considered in the M2M system. Therefore, fairly many stations may be connected to one AP (access point).
2) Low traffic load per station: in general, the M2M terminal has traffic patterns of collecting and reporting peripheral information such that there is no need to frequently transmit data and the amount of data to be transmitted is also small.
3) Uplink is main: a M2M network mainly receives commands by downlink and has the structure of reporting resulting data by uplink after taking actions. Since main data are usually transmitted by uplink, the uplink is main in the M2M network.
4) Long-life station: the M2M terminal is mainly operated by a battery, and there are many cases difficult for an user to frequently charge the battery. Therefore, The M2M terminal are required to minimize battery consumption to ensure long life.
5) Automatic recovering function: the M2M terminal is difficult to directly operate by the man in a specific situation, and therefore, the function of recovering the terminal for oneself is required.
The distinctive feature of IEEE 802.11 ah is that a non-permission band of sub 1 GHz except a TV white space band may be used as the frequency band. On using sub 1 GHz band, the coverage in the network may have a fairly wide coverage (up to 1 km) as compared with the WLAN focused on the existing indoor facility. That is, unlike the existing WLAN frequency band, that is, 2.4 GHz or 5 GHz, when the WLAN is used in the frequency band of sub 1 GHz representative of 700-900 MHz, the coverage of the AP is extended by two to three times as compared with the same transmitting power due to radio wave characteristic of the corresponding band. Due to this characteristic, it is assumed that fairly many STAs per one AP may be connected in 802.11 ah standard.
The use cases considered in IEEE 802.11 ah standard may include cases below.
Use Case 1: Sensors and meters
1a: Smart Grid—Meter to Pole
1c: Environmental/Agricultural Monitoring
1d: Industrial process sensors
1e: Healthcare
1f: Healthcare
1g: Home/Building Automation
1h: Home sensors
Use Case 2: Backhaul Sensor and meter data
Backhaul aggregation of sensors
Backhaul aggregation of industrial sensors
Use Case 3: Extended range Wi-Fi
Outdoor extended range hotspot
Outdoor Wi-Fi for cellular traffic offloading
The case of Use Case 1, that is, Sensors and meters is the use case related to M2M communication described above, and in this case, various kinds of sensor devices are connected to the AP using 802.11 ah standard to perform the M2M communication. Particularly, in case of a smart grid, the sensor devices up to 6000 may be connected to one AP.
In case of Use Case 2, that is, Backhaul Sensors and meter data, 802.11 ah AP providing a wide coverage functions to link to a backhaul of a different system such as 802.15.4g.
The Use Case 3 includes the case performing an outdoor extended range hotspot communication such as an Extended home coverage, a campus wide coverage, shopping malls, and the case distributing heavy-loaded cellular traffics by supporting traffic offloading of cellular mobile communication by 802.11ah AP.
The frame structure should be used in 2/4/8/16/8+8 MHz channel bandwidth at the frequency band at 802.11ah standard, that is, sub 1 GHz band. The frame structure that may be used at this frequency band may use the structure for simply 1/10 down-clocking PPDU frames used at the existing IEEE 802.11 ac standard. That is, the frame format used in 20/40/80/160/80+80 MHz channel bandwidth at 802.11ac standard is 1/10 down-clocked to generate the frame format used in 2/4/8/16/8+8 MHz channel bandwidth at sub 1 GHz band.